i. Technical Field
The apparatus of the present invention relates generally to the field of sorting operations, and relates more particularly to an apparatus for separating a continuous stream of wood chips into separate fractions based on chip size. Specifically, the invention pertains to a wood particle screen particularly useful for separating small particles, or fines, from the larger, acceptable wood chips.
ii. Prior Art
In the papermaking process, wood chips are cut from pulp wood logs, and are cooked in digesters to remove lignin and release the cellulose fibers for paper production. For optimum digester operation and pulp production, it is desirable to control the proportions of oversize and undersize wood chip fractions placed into the digesters. Therefore, various types of screening devices have been used in the past to separate oversize wood chips, which may be undercooked in a digester, from chips within the acceptable size range; and also for removing undersized chips, which may be overcooked in the digester, from the acceptable fraction. The oversized can be sliced and returned, while the undersized having useful fiber can be returned in acceptable proportions through metered flow.
The smallest size particles in a chip stream are commonly referred to as fines, and, while the precise definition of fines may vary from pulp mill to pulp mill, fines can generally be described as the dust-like material having little useful fiber. In most operations, fines are undesirable and complete removal is desirable. However, the slightly larger material, frequently called pins, has useful fiber and can be tolerated in acceptable proportions. Frequently, the distinction between fines and pins is determined by screening through a 3 millimeter round hole, that material passing through the hole being fines.
A widely-accepted type of screen used for separating wood chips into fractions based on chip thickness is a disk screen. In a disk screen, a plurality of parallel shafts have spaced disks thereon, and the disks from adjacent shafts interleave with one another to define interdisk facial openings (I.F.O.s). Material smaller than the screen I.F.O. passes through the screen, while the larger material carries over the screen. The contour of the peripheries of the disks, the disk size and shaft rotational speed can be selected to agitate the chip stream as aggressively as desired. Chip agitation will tend to break-up clumps, reorient chips and sift the smaller particles from larger chips.
Disk screens have been widely used for various types of screening, such as removing oversize or undersize chips, utilizing I.F.O.'s of various dimensions. In some screening operations, disk screens are arranged substantially flat, while in other operations, commonly referred to as V-screens, two disk screen beds are angled upwardly. Typically, on flat disk screens the general direction of chip flow is perpendicular to the rotatable shafts, and in V-screens the chip flow is generally parallel to the shafts.
While disk screens can be assembled with I.F.O.'s and chip agitation which result in very high fine removal efficiency, a substantial portion of the slightly larger fiber material, including pins, will also pass through the disk screen. High pin loss in such systems is undesirable, and separate fines/pins classification may be required following a disk screen used for fines removal.
Shaker screens are also used for fines removal, and generally consist of a rigid plate, frequently disposed at an angle, through which holes are provided. The plate is movably mounted on springs or pivotable suspensions. A drive is provided to move or shake the plate, often in an orbital path or in linear oscillation, to slide the chips along the plate surface, thereby enabling fines to pass through the openings. Holes in the shaker screen must be quite small, on the order of 3 millimeters diameter, to reduce the loss of acceptable fibers such as those contained in pins. In inclement weather, snow, ice, and the like can fill the openings, as can dirt, mud, and other grime frequently found in pulp mill operations. Small wood chips also can become wedged in the openings, thereby preventing fines from passing therethrough. When a substantial portion of the holes in a shaker screen become blinded by chips, dirt, snow, or the like, the fines removal efficiency of the screen decreases dramatically.
The generally planar movement of the shaker screen plate, either orbital or linearly reciprocating, does not aggressively agitate the wood chips, and fines clinging to the chips frequently will not be dislodged therefrom. Therefore, while pin chip loss in a shaker screen may be less than for disk screens, the fines removal efficiency, even under optimal operating conditions is not as high for shaker screens as for disk screens, and the efficiency decreases still further when blinding occurs. A further disadvantage of shaker screens is that the reciprocating or orbital movement of the heavy shaker plate creates excessive vibration, requiring compensation in the structure of the screen unit.
Modified shaker screens having screen beds of flexible material, such as polyurethane, have been used with intermittent supports beneath the deck being movable to cause intermediate portions of the deck between adjacent supports to flex or buckle. The shape of the deck between supports changes from a substantially flat to depressed or concave. While the flexible screening beds, like the rigid shaker screens, substantially reduce the pin chip loss, blinding remains a problem, and separation of fines which are adhered to acceptable chips is not as efficient as in disk screens. Substantial nonactive screening areas exist along the regions of the support bars. While perhaps exceeding conventional shaker screens, the overall fines removal efficiency generally is not as high for this type of screen as for disk screens, and falls even lower when blinding occurs.
Other types of winnowing apparatus using air flow to separate fractions have been used for screening wood chips, but tend to separate more on mass and aerodynamic properties than do other types of screens. Efficiency, therefore, is less. Rotating separators applying centrifugal force to the chips have also been used, but with mixed results.